The present invention relates to an integrated circuit architecture for CMOS imagers. More specifically, the present invention relates to methods and apparatus for masking or correcting faults in individual pixels of CMOS imagers.
CMOS image sensors are now becoming competitive with charge coupled device (xe2x80x9cCCDxe2x80x9d) array image sensors. Potential applications include digital cameras, night time driving displays for automobiles, and computer peripherals for document capture and visual communications.
Since the 1970s, CCD arrays have dominated the electronic image sensor market. They have outperformed CMOS array sensors in most areas, including quantum efficiency, optical fill factor (the fraction of a pixel used for detection), charge transfer efficiency, readout rate, readout noise, and dynamic range. However, the steady improvement in CMOS technology (including increasingly small device size) has moved CMOS image sensors into a competitive posture. Further, in comparison to CCD technology, CMOS technology provides lower power consumption, increased functionality, and potentially lower cost. Researchers now envision single chip CMOS cameras having (a) integrated timing and control electronics, (b) a sensor array, (c) signal processing electronics, (d) an analog-to-digital converter, and (e) interface electronics. See Fossum, xe2x80x9cCMOS Image Sensors: Electronic Camera On A Chip,xe2x80x9d 1995 IEDM Technical Digest, Wash. D.C., Dec. 10-13, 1995, pp. 17-25 which is incorporated herein by reference for all purposes.
CCD arrays are limited in that all image data is read by shifting analog charge packets from the CCD array interior to the periphery in a pixel-by-pixel manner. Unfortunately, the pixels of the CCD array are not randomly addressable. In addition, due to voltage, capacitance, and process constraints, CCD arrays are not well suited to integration at the level possible in CMOS integrated circuits. Hence, any supplemental processing circuitry required for CCD sensors (e.g., memory for storing information related to the sensor) must generally be provided on separate chips. This, of course, increases the system""s cost.
A persistent problem of both CMOS and CCD image sensor technologies is image degradation due to faulty pixels. Such faulty pixels arise from processing variations inherent in fabrication of numerous sensor chips. A pixel""s fault may be manifested by an output indicative of a radiation exposure that does not accurately reflect the actual radiation exposure to which the pixel was exposed. For example, a pixel that outputs more charge than is expected upon exposure to a particular amount of radiation appears as a bright spot in an image. Similarly, a pixel that outputs less charge than expected appears as a dark spot.
Typically, image sensors are tested after fabrication to identify the number of faulty pixels that they contain. If any sensor has more than a specified number of faulty pixels, it must be rejected. Thus, sensor yield is limited by the number of faulty pixels typically produced on a chip. Not surprisingly, wide area sensors having large numbers of pixels have relatively low yields because they tend to have higher numbers of faulty pixels (the number of faulty pixels per total number of pixels is approximately constant for a given fabrication technology).
While careful screening of image sensors after fabrication can locate defective arrays, it cannot prevent sensors from degrading during normal use. Pixels sometimes develop such faults during normal operation. Unfortunately, no effective mechanism exists for identifying and correcting such faults.
What is needed therefore is an improved image sensor that can mask or otherwise correct defective pixels soon after the sensor is fabricated and during its lifetime.
The present invention provides a fault tolerant radiation imager such as a CMOS imager. Such image sensor includes circuitry for masking and/or correcting defective pixels during image generation. Masking may involve, in one example, replacing the output of a given pixel with an average of the outputs of surrounding non-faulty pixels. Correction may involve increasing or decreasing the output level from a pixel to be corrected. Thus, while image sensors may be fabricated with some number of faulty pixels, the images produced by such sensors will be superior to those produced by sensors that do not have such masking or correcting mechanisms. Further, sensor yield.may be improved because even those sensors having more than the traditionally allotted number of defects may be used (so long as the defective pixels are appropriately masked or corrected). In one preferred embodiment, pixels that were initially good and later become defective may be identified at the time they become defective. The newly defective pixels so identified may then be masked or corrected as described to thereby increase the CMOS detector lifetime.
One aspect of the present invention provides a method of masking faulty pixels that may be characterized by the following steps: (a) exposing an imager to radiation; (b) identifying a faulty pixel in the imager; (c) determining outputs of a plurality of other pixels located about the faulty pixel; and (d) masking an output of the faulty pixel using the outputs of the plurality of other pixels. Then a masked output of the faulty pixel can be provided in an image produced by the imager. Preferably, the system identifies the faulty pixel by accessing memory associated with the imager to determine the location of the faulty pixel in the imager. Then to generate an image corrected for the faulty pixel, the method may also require conversion of the output of the pixels (including the masked output) from an analog format to a digital format prior to masking the pixel""s output.
In one embodiment, masking the output of a faulty pixel involves replacing its output with an output value based exclusively on the output of multiple surrounding pixels. For example, the corrected output may be obtained by interpolating output values of at least two pixels surrounding the at least one faulty pixel. In color imagers, the pixels of the imager can discriminate between multiple colors and the faulty pixel therefore will be designated to detect radiation of a single color. Preferably, in such cases, the plurality of other pixels chosen to mask the output of the faulty pixel are only those surrounding pixels that are designated to detect radiation of the same color as the faulty pixel.
It may not be necessary to totally mask the output of each of the faulty pixels. For example, some pixels may not be completely corrupted, in which case their outputs could be corrected rather than masked. In one instance, the faulty pixel may be known to have an output that is 20% lower than appropriate. Thus, the output of that pixel may simply be increased by 25% before producing the image.
Of course, some technique for identifying faulty pixels must be conducted initially (when the imager is produced) or periodically over its lifetime. In one embodiment, the system electronically tests a selected pixel according to the following sequence: (i) electronically resetting the selected pixel to a defined charge (e.g., the charge associated with a dark value in an image); (ii) reading the selected pixel""s output; and (iii) comparing the selected pixel""s output to an expected value based upon the defined charge provided to the selected pixel. If the selected pixel""s output significantly deviates from the expected value, the system designates the selected pixel as faulty. Alternatively, the conducted test may also be an optical test in which the imager is exposed to radiation of a known intensity for a known duration. The outputs of the pixels are then compared to expected values as described above.
In the case of an electronic test and where the selected pixel includes an n-well and a p-diffusion photodiode, electronically resetting the selected pixel requires injecting a quantity of negative charge into the p-diffusion. In photodiodes of the opposite conductivity type, negative charge is removed from an n-type diffusion during reset.
The test results may be interpreted as follows: if the selected pixel""s output significantly deviates from the expected value, the system determines whether the selected pixel is partially or completely corrupted. Thereafter, masking or correcting the output of the selected pixel employs a first technique for a partially corrupted pixel and a second technique for a completely corrupted pixel. Because the second technique involves a completely corrupted pixel, little if any information from the output of the pixel may be used. Thus, the second masking technique may involve interpolation or otherwise averaging the outputs of pixels surrounding the completely corrupted pixel. The first technique may involve using the output from the pixelxe2x80x94simply increasing or decreasing that output by a fixed percentage or magnitude.
To automatically recall the faulty pixel identities prior to image generation, the test method may be concluded by storing the location of the one or more faulty pixels in a memory. In appropriate instances, it may also involve storing data specifying whether each of the one or more faulty pixels is partially or completely corrupted.
Another aspect of the invention provides an imager that may be characterized as including the following elements: (a) one or more pixels capable of providing outputs indicative of a quantity or a type or both the quantity and type of radiation to which each of the one or more pixels has been exposed; (b) one or more circuit elements electrically coupled to the one or more pixels and configured to mask or correct faulty pixels in the imager; and (c) a memory configured to store the locations of the faulty pixels. Preferably, these various elements are provided on a single integrated circuit chip.
The imager may also include one or more voltage sources capable of delivering a voltage to the photodiode diffusions of the one or more pixels (typically to reset the pixels"" states). It may further include one or more charge integrators capable of measuring the pixel outputs. Still further it may include an analog-to-digital converter capable of receiving analog outputs from the one or more pixels, converting the analog outputs to digital signals, and providing the digital signals to the one or more circuit elements such that the one or more circuit elements can identify faulty pixels. In an especially preferred embodiment, the plurality of pixels is arranged in an array such that each of the pixels in the plurality of pixels is separately addressable.
In many CMOS imagers, the one more pixels will each include a photodiode diffusion formed in a well and a tap to power or ground also formed in the well. Such pixels may be of an active or passive design.
Another aspect of the invention provides a system for producing an image of an object. This system includes an imager of the type described above and one or more components for outputting an image resulting from the outputs of the one or more pixels. The image may be a photograph in the case of a digital camera for example.
These and other features and advantages of the invention will be described below in the Detailed Description section with reference to the appended drawings.